Various methods have been employed to produce aralkyl isocyanate compounds. It is known for example, that organic diamine or monoamine derivatives can be reacted with phosgene to produce corresponding mono- and diisocyanates.
Drawbacks with phosgenation reactions include the toxicity of phosgene, and corrosion problems associated with the by-product hydrochloric acid. Furthermore, the organic amines are difficult to prepare requiring elevated pressures while producing low yields of final product.
The difficulties with direct phosgenation have led to the development of non-phosgenation routes.
In particular, tertiary aralkyl isocyanates have been prepared by a variety of different reactions, such as by reaction of corresponding olefins with isocyanic acid as described in U.S. Pat. No. 3,290,350, and by reaction of the corresponding halides with an alkali metal isocyanate as disclosed in U.S. Pat. No. 4,130,577. The olefin addition route suffers the disadvantages that the yields are poor and that large amounts of olefin and isocyanic acid are lost through self polymerization. On the other hand while the reaction of the halide with the alkali metal isocyanate can provide high yields, the reaction times are long, and the halogen is completely lost as the alkali metal halide, recoverable only at great expense. The reaction of the halide with isocyanic acid is a substantial improvement in terms of yield and reaction time over the prior processes but it consumes hydrogen halide in the manufacture of the aromatic halides from the corresponding olefins.
Other investigations have reported the preparation of aralkyl isocyanates by reaction of the corresponding halide with isocyanic acid as in U.S. Pat. No. 4,361,518, by addition of the corresponding olefin with a carbamoyl halide to form the benzyl halide followed by reaction with an excess of isocyanic acid as disclosed in U.S. Pat. No. 4,379,767, and by thermal cracking of urethanes formed by reaction with corresponding olefins and carbamic acid esters as in U.S. Pat. No. 4,439,616.
Aralkyl urethanes, which can be used as intermediates in the synthesis of isocyanate derivatives, have likewise been produced by a number of routes.
Merely by way of illustration, Singh et al., U.S. Pat. No. 4,570,012 report the production of tertiary aralkyl urethanes, such as tetramethyl xylylene diurethanes, by addition of corresponding tertiary aromatic diols and carbamic acids at moderate temperatures and in the presence of acid catalyst. This method of production is disadvantageous in that it requires expensive aromatic diol compounds which are not widely available commercially.
It has been surprisingly discovered now that primary aralkyl urethanes and ureas and corresponding isocyanate compounds, as well as aliphatic isocyanate compounds, can be synthesized by a new and novel route that utilizes low cost, readily available commodity chemicals such as toluene, xylene, urea, methanol and formaldehyde and yet, still avoids the conventional toxic phosgenation technology.